How do I change the linear stiffness of an object programmatically?
If you simply want to batch-modify physics values on objects, you could do something like this:

import bpy

for obj in bpy.context.selected_objects:
    if obj.game.physics_type == 'SOFT_BODY':
        obj.game.soft_body.linear_stiffness += 0.1 #or whatever other operation


I'm not really familiar with the BGE, though.